From the Literature: Mass Migrations in Dragonflies

Wow! I’m running way behind on getting a new post up. I’ve been getting a ton of dragonfly swarm reports in the last week, so I’ve been scrambling around trying to keep up with that. Couple that with some writer’s block and the start of my field season, and Ms. Dragonfly’s life has been a bit hectic! I’ll try to get something up a bit more quickly next time…

I’m going to do a short series on dragonfly swarming behavior since this seems to be a topic that a lot of people are interested in this summer – and one that I’ve become rather obsessed with over the last few months. (Seriously – ask my husband, my Ph.D. advisor, or my coworkers how many times I’ve brought up dragonfly swarms in conversation over the last few weeks!) After combing through the literature, I’ve discovered some great information on dragonfly swarming and I think I have some good explanations for why we see these swarms in nature. Today’s post will cover a paper that came out in 1998 about the mass migratory swarms observed in dragonflies. For those of you out there who have seen hundreds of thousands or millions of dragonflies all flying together in a single direction, a sort of river of dragonflies, this paper describes and attempts to explain this behavior. And what an amazing behavior it is! I would dearly love to see one of these giant migratory swarms. If any of you happen to get this behavior on video, I strongly encourage you to upload it to YouTube so that others and I can see it! But let’s get to that paper.

Over the years, several dragonfly researchers have witnessed massive swarms of dragonflies flying together in what appeared to be migratory swarms. Each time, they consisted of several hundreds of thousands or millions of dragonflies, enough to nearly blacken the sky and make themselves really obvious – if you happened to be looking up at the right time to see them! In 1998, Robert Russell, Michael May, Kenneth Soltesz, and John Fitzpatrick combined their eyewitness accounts of these migratory swarms and wrote one of the first papers describing this behavior in detail. At the time of the paper’s release, very little was known about dragonfly migratory patterns. 25-50 species of all dragonflies were thought to be migratory and only 18 migratory species had been reported in the eastern United States. In their paper, the authors set out to describe the massive swarm migrations they’d observed in dragonflies, but they also wanted to review the literature concerning migrations of dragonflies to see if any patterns emerged. They were hoping to determine why these swarms formed and understand how they worked.

First the authors described three swarm migrations in the United States that three of the four authors had witnessed themselves: one swarm each in Chicago, Illinois; Cape May, New Jersey; and Crescent Beach, Florida. In each place, the observers reported the same things:

Nearly all of the dragonflies in each swarm were moving together in the same direction, though not all swarms moved the same direction.

The swarms were largely made up of green darners (Anax junius – pictured to the left above), though several other species were present in the Cape May swarm to a much lesser extent (black saddlebags, twelve-spotted skimmers, swamp darners, Carolina saddlebags, wandering gliders, spot-winged gliders, and blue dashers).

The swarms were very large, at least 200,000 dragonflies (Florida) and up to an estimated 1.2 million in the Chicago swarm! (They compared the dragonfly swarms they observed to those observed in locusts and found that the number of swarming dragonflies was comparable to swarming locusts. If you’ve heard about locust plagues, you have a good idea of what one of these swarms should look like!)

The swarms tended to be very compact, that is they flew within a rather confined corridor rather than spreading far apart either horizontally or vertically.

The swarms tended to follow very obvious landmarks such as shorelines, ridges, and coasts.

All of the swarms took place soon after a cold front passed through the area, so they appeared to be distinctly weather-related.

The authors also reviewed the literature and collected field observations from other researchers to better determine how, where, and when these massive swarms were taking place. They gathered 41 reports spanning the years 1881 through 1995 and found that these swarms generally shared the same characteristics as the swarms witnessed by the authors. They further discovered that the swarms took place between July 30 and October 13, though the majority (28 swarms) occurred in September. This was clearly a highly seasonal event. All of these reports were also made in locations in the eastern United States, so it appeared that these swarms only occurred in half of the U.S.

Based on all of the reports together, the authors made some speculations about the massive migratory swarming behavior observed in several species of dragonflies. First, their data suggest that at least some of the dragonfly species are migratory and are using these swarms to travel from north to south during the fall. Where exactly they’re going is much less certain, but there are some reports of dragonflies crossing the Gulf of Mexico and massive swarms suddenly appearing in locations in Mexico. The final destination could be as far south as Central America or as far north as the Gulf coast. Regardless of where they end up, the dragonflies appear to be flying south for the winter, much like birds and monarchs do.

Second, the authors suggest that this type of swarming behavior might be unique to the United States and the regions where the dragonflies overwinter. There are similar swarms in Europe, but the timing and recurrence of the swarms are very different than the yearly migrations observed in the US. This also led the authors to suggest that the wandering glider, the only dragonfly species naturally found on all continents, might not form these migratory swarms. While they have been observed in American migratory swarms, they typically only make up a small percentage of the total individuals within the swarm and are very uncommon in swarms reported in other locations.

Third, the authors proposed that weather may be the most important factor regulating the mass migrations of dragonflies. Indeed, in nearly all of the accounts they collected, a cold front had passed through the area just prior to the appearance of the swarm. A combination of season (autumn), north-westerly winds, and cold fronts may be necessary to entice dragonflies to fly from the cooler northern regions to warmer southern climates for the winter. The timing of this behavior also suggests that these dragonflies are relying on aerial plankton for food as they migrate. Aerial plankton is abundant during the fall, allowing the dragonflies to feed along the way.

Fourth, landmarks appear to be very important in directing dragonfly migration paths. Mass migrations are almost always reported along major waterway, coasts, or prominent land features. These “leading lines” help dragonflies orient themselves and follow the correct path south. However, they also seem to reorient themselves under certain conditions, especially when they have to cross significant stretches of water. This will be the subject of an upcoming post, so I won’t go into more detail here.

And last, there is some evidence that the dragonflies that fly south for the winter are not the same individuals that return north. Dragonfly adults typically have a life span far shorter than the length of time the dragonflies overwinter, so it is very likely that migratory dragonflies fly south, mate, produce young, and die. The offspring then make the return trip north in the spring. There is a lot of uncertainty about what happens in the spring, however. The route the dragonflies take on the return trip remains mysterious. The migration north also does not appear to involve huge migratory swarms and may be comparatively inconspicuous, making it difficult to determine when and where the northward bound dragonflies are flying.

The authors ended their paper with a long list of questions that remain unknown about dragonfly migration. 12 years later, we still don’t know most of the answers to these questions! For example, why exactly are these dragonflies forming these massive swarms when they migrate? If it were only to protect the dragonflies as they fly (safety in numbers), then why do so few dragonfly species exhibit this behavior? It doesn’t appear to be solely related to timing of adult emergence from the water either as emergence is not synchronous in the green darners and they are the most commonly observed migratory species. Still, this paper went a long way toward explaining mass migrations in dragonflies. Due to the ephemeral nature of swarms – a million dragonflies might fly over an area in a matter of minutes – we’re lucky there have been any studies of this behavior at all!

Over the next few posts, I’ll go over another paper about dragonfly migrations (one using radio transmitters!) and talk about what I’ve discovered in the literature about what I’m calling static swarms. With these three posts, I hope to summarize most of what’s known about dragonfly swarming behaviors so that the information is available outside of the scientific community in an accessible way. I am also going to post photos of all of the known migratory dragonfly species in the US. Many people reporting dragonfly swarms to me have expressed an interest in identifying the dragonflies in their swarm, so I want to make that possible. And, as always, keep sending me dragonfly swarm reports! I have a huge collection of them going this summer and I couldn’t be happier with the level of participation in my project.

I am tracking swarms so I can learn more about this interesting behavior. If you see one, I’d love to hear from you! Please visit my Report a Dragonfly Swarm page to fill out the official report form. It only takes a few minutes!

This is my biggest pet peeve about science: that we, as scientists, have next to no ability to communicate what we know with the rest of the world. I think this causes scientific illiteracy and a massive disconnect between the people who discover new things (scientists) and the people who generally make the policy decisions (the non-scientist public). It doesn’t help that scientists effectively speak their own language so that the average person simply CAN’T read our work. Non-scientists who are motivated to do so COULD learn the language, but it’s not a trivial thing to do. Couple that with the fact that most scientific journals are not accessible outside university libraries and the majority of the vast wealth of knowledge that scientists have learned remains all but hidden from non-scientists. I hate this state of affairs. It’s just terrible that this is how science works. Shouldn’t anyone be able to look up information about dragonfly swarms? Apparently not. So, I try to make my one little corner of the world, the science that I do myself or am familiar with/interested in, accessible to everyone. I’m glad that you agree with me that this is something that more scientists should do!

When I found this dragonfly dying at the side of the road in the middle of June, I thought that it was probably diseased since I figured that if it had emerged from local waters, it would be too young to be dying of old age. But, from what you say here, maybe it had migrated up from considerably further south, in which case it could have been actually pretty old?

Up until now, I had thought that up here in northern Michigan, dragonfly nymphs just overwintered under the ice. Do some green darners overwinter locally while others fly south, or are they generally migratory? Or is this one of these questions that nobody knows the answer to yet?

It’s quite possible that you might have found a migratory individual, though what I’ve read about darners recently suggest that they sometimes emerge very early in the year, before everything else. Their flight season is supposedly April-Nov in the north. And yes, it does look like there are two distinct types of the darners. Some overwinter in place, some are migratory. Odd that only part of the population migrates, don’t you think? That’s still an unanswered question, why they don’t all migrate.

I passed through a swarm going north over a highway near Pella ,IA for about 6 miles on 8-14 at 6pm. It was amazing how they were everywhere.I wasn’t sure of species, I thought bigger than a darner.I wondered about a migration but they were flying N not S.None ever hit my vehicle.

Last evening on 8-17-10 was witness to a swarm of dragonflies here in Spokane Washington. Not sure if they were headed anywhere, they were just here for about an a couple hours and then when the sun was setting seemed to disappear. I had to go to the store and that was about 2 miles away and i encountered them the entire time. There was places that they were thicker than others and a few city blocks that I did not see them at all. I have never seen such a swarm of dragonflies so it was interesting to witness.

just had a swarm fly over my house in wilmington nc less than a mile from the beach. there is a thunderstorm coming and we also had a storm last night. it is 81 degrees now and this happened about 15 minutes ago at 7:20 pm on august 23 2010. not sure which species but some did dip down and we ducked. they were going over for about 10-15 minutes straight.

There are swarms going over us right now in Punta Gorda, Belize. I found your site while I was looking for some explanation of this. My camera isn’t good enough to capture it but I tried. They were going south to north. I have begun asking the folks around here if it has any correlation to the weather, but I haven’t received an answer yet. Very interesting sight to see.

I don’t think you got the post of the siting of the August 10 that I typed earlier today. For 100 miles on I-70 we saw one huge swarm that was in the 10’s of thousands that we saw and who knows how far North and South the swarm was. They were huge dragon flies too. Awesome and something we had never seen…. In kansas coming back from Colorado.

I have seen that paper and I think it’s really interesting. And the hawk counting stations are a great place to look for dragonflies! I wonder if I can get in touch with some of them…

Your area has had an absolute explosion of dragonflies recently, so I hope you are being treated to a spectacular show this summer! It really makes me wish I lived in your area (though being a desert gal, I would likely freeze).

hawkcount.org has all the listing for the counting stations. I think Hawk Ridge (www.hawkridge.org) would be a good one because Lake Superior sort of funnels all the birds through that area. The staff are very friendly also. Good luck!

The dragonflies have been amazing this year. We do have many seasonal human residents (a.k.a. “snowbirds”) that live up here in the summer and head south for the colder months. You could be a “snowdragonfly!”

HI
ON 9/11/10 I SAW A HUGE SWARM OF GREEN DARNERS-100S IN THE AREA OF 30-60 ATLANTIC AVE WESTERLY RI (WATCH HILL END) 02861 ACROSS FROM THE WATER NEAR A STAND OF BLACK PINES.THERE WERE HUNDREDS OF THEM.IT WAS A BIT WINDY AND THEY STAYED HOVERING FOR ABOUT AN HR.IT WAS A VERY IMPRESSIVE SIGHT. THE PREVIOUS NITE HAD BEEN COOL. UP UNTIL THAT POINT IT WAS UNUSUALLY WARM
THE AREA IS ALSO USED AS A FLYWAY FOR MONARCHS AND OTHER MIGRATING BIRDS. .

There was a very big swarm on the Belizean coast. On the morning of Sept. 03 after watching the sunrise at 05:45 (after a night thunderstorm around 02:10 continuing to 03:30) we walked back from the pier at Hamanasi resort to the guest house (05:50) . As we approached the guest house we saw a “river” of dragonflies flying North, parallel to the beach, about 400 feet from the shoreline. They were in a “column” about 20 feet by 20 feet wide and all moving in the same direction except for 1 or 2 odd ones that seemed to be flying counter-current that I pointed out to my girlfriend. There was a clear line of foliage between them and the beach. They had clear wings with distinctive dark marks on the tips. (as if their wingtips had been dipped in ink). I caught flashes of blue coloring but I couldn’t tell what part of the body it was from. We proceeded to our cabin, showered, and returned for breakfast to the same guesthouse and while sitting on the porch and eating we continued to observe the swarm. Other guests came out and pointed and observed and we all commented but oddly, none of us took pictures. This was 07:00 or so by then. I was curious as the t-shirts of the Hamanasi staff have a big dragonfly printed on the back and I was wondering what the significance was and if this swarm was related. We retired from breakfast to our room and got ready for a trip to some Mayan ruins and while gathering to meet the van the dragonfly swarm was still proceeding. As we headed out of Hamanasi and North towards Hopkins I could still see the swarm flying North at 08:30. There must have been millions of dragonflies that had passed that location by that time. We then turned West on our trip so I lost sight of the dragonflies but still wondered what that was about.

Last night, September 17th, 2010, a swarm of dragonflies appeared over our house in Omaha, NE. We are close to the Missouri River – about 3 or 4 miles. It was right around dusk – the sun was setting. I’m unsure of the type of dragonfly, but they appeared to be larger than average. It was a very warm day, but a cold front was coming in and storms started up later. They flew almost exactly north to south, flying just above the roof line, with the occasional dragonfly coming down to investigate our little party. They weren’t terribly close together, maybe 8 to 10 inches on average. From when we noticed the swarm to the end was about 5 minutes – they stretched as far as we could see west to east. I’m no good at estimating numbers, so I won’t even try but it was definitely more than a few thousand. It was a very mosquito-filled evening as well. Thank you for your information – if I’d been alone I would have thought I was imagining things – and your site would have confirmed my sanity.

I’m staying on st george island off the panhandle of fla and for the past few days there have been large swarms of dragonflies flying east about 4 stories up. 7 – 8 am and 7-8 pm they aren’t flying double tho! They are the large green ones. Very cool!

We are at an RV park just east of Carrabelle, Florida (N. 29 degree, 25min 16.2 sec/W084degree,36min 33.2 sec) and saw a swarm of, what looked like the same, ie. Green Darner, Dragonflies flying east at about 7.30pm which lasted for at least an hour. There were thousands in the swarm. The were flying over the edge of the water (we were stood on a fishing pier). Some people stood with us said the same thing had happened a few days ago. I will try and video it if it happens tonight. The weather was still, calm, no wind. The water was calm as a millpond. Windy conditions later but no storm.

September 28th, 2010-6pm -8pm. Cape San Blas, Florida. It rained yesterday.
Temperature today in the high eighties. Traveling north along the beach. Millions of them, they were still travelling at 8pm when I went inside.

Galveston Texas, October 10-11 2009, noticed southward migration of tens of thousands of individual dragonflies for about two days (may have gone on before and after we left). Flying over dunes at heights of near surface to perhaps ten meters, estimated two or three per second passing line of sight for an estimated 100 to 200 per minute or nearly 1,000 per hour. At least ten hours directly observed for a minimum of 10,000 individuals over two day period in a narrow band at the coast’s edge. Unknown if migrations were also occurring on other parts of the island, or just on coast. Beer ran out and had to cease surveying.

We just returned from Cancun, MX. At our resort, for two days, there were hundreds of dragonflies on the glass walls and up on our balcony. Then the third day there were none to be seen at all. They may have been heading north and got hung up due to the structure of the hotel impeding their way? I could get the ones on our balcony to sit right on my finger. It was amazing.

We are visiting our condo in Destin, FL and are amazed at the amount of dragonflies here! They are everywhere! When we walked our dogs last night they were resting on the grass and flew into my husband’s head!

We live north of Atlanta and several weeks ago I watched several dragonflies weaving in and out of swarming bugs in my backyard. I did a little research and found out they are very carnivorous.

Why are there so many of these beautiful creatures all of a sudden? Are they swarming?

Tropical Storm Isaac is heading this way and I fear they will be disseminated!

Sounds like you’ve been seeing dragonfly swarms! There was a big surge in the swarming activity in Florida as Isaac approached and I suspect that it was the cause of the frenzy of dragonflies in the area. I was actually in Florida at the same time you were and was seeing the same thing! I thought it was fantastic. Why, specifically, they’re there… That’s a little less certain. It could be that the dragonflies are taking advantage of insects that were blown in by the approaching storm. It could also be that the dragonflies themselves were being blown in by the storm and then formed feeding swarms as they traveled north. I have limited evidence for the latter, so I suspect that the dragonflies were traveling in front of the storm.

I live in delray beach Florida and today out of nowhere there were suddenly dragonflies everywhere I started to notice a couple early in the day and by the time evening rolled around you could look up and see hundreds they were not flying in any direction just milling about I guess but for sure that was the first time even saw a dragonfly here and I have been in Florida since may. Do you think the swarm stopped to eat and rest?

At this time of year it was likely just a feeding swarm and not a mass migration, unless you’ve all had some really nasty storms already, or have some coming toward you. Don’t suppose you’d be willing to submit a report of your swarm to my dragonfly swarm project? I’d love to document your swarm, so the form’s located here: https://thedragonflywoman.com/dsp/report/. Thanks!